Field of the Invention
The present invention relates to an optical transmitter, and more particularly, to an optical transmitter that amplifies a modulated light beam with a semiconductor optical amplifier.
Description of the Background Art
Examples of modulation techniques of dual polarization with a Mach-Zehnder (MZ) optical modulator include dual polarization-quadrature phase shift keying (DP-QPSK) and dual polarization quadrature amplitude modulation (DP-16QAM). The modulation techniques have a great loss, so that a configuration is known that includes a semiconductor optical amplifier (SOA) located in a subsequent stage of a modulator to increase optical output.
A configuration of a conventional optical transmitter will be described. A laser light source is split into two, which are assumed to be an X polarized wave and a Y polarized wave. After the MZ optical modulators each modulate the polarized waves, an output of the Y polarized wave is rotated by a π/2 polarized wave, and the X polarized wave and the Y polarized wave are polarized and combined. After they are polarized and combined, the SOA in the subsequent stage amplifies the output modulated signal (for example, see Japanese Patent Application Laid-Open No. 2011-188213).
A modulation loss in the MZ optical modulators, the presence or absence of a polarization rotation, or the like causes a difference in light output intensity between the X polarized wave and the Y polarized wave. Thus, a detector that detects the light output intensity is provided in each of the subsequent stages of the two MZ optical modulators, and the information is used to control a gain of each of the polarized waves in the SOA.
An optical signal transmitting device disclosed in Japanese Patent Application Laid-Open No. 2011-188213 has a configuration only applicable to an SOA that amplifies two polarized waves orthogonal to each other. Further, in a case where the information about the detected power of each of the polarized waves input to the SOA is used to control the gain of the SOA, the polarized waves may vary in the intensity since a relationship between the current flowing through the SOA and the gain actually varies with, for example, ambient temperature and individual differences of the SOA elements. The configuration for detecting the output intensity of the SOA also needs a polarization separator and a polarization rotating unit in the detector, resulting in the complicated configuration.